1. Field of the Invention
This invention relates to an exposure system for exposing an image recording medium to a recording light by scanning the image recording medium with the recording light to read the image recorded on the recording medium, and more particularly to such an exposure system having a hole array.
2. Description of the Related Art
There has been known an image reading system in which a photoconductive body such as a selenium plate sensitive to X-rays is employed as an electrostatic recording medium, the electrostatic recording medium is exposed to radiation such as X-rays carrying thereon radiation image information to store a latent image electric charge representing the radiation image, and then the radiation image information is read out by scanning the electrostatic recording medium with a laser beam and detecting the electric current generated in the electrostatic recording medium upon exposure to the laser beam through plane electrodes or stripe electrodes on opposite sides of the electrostatic recording medium. See, for instance, U.S. Pat. Nos. 4,176,275, 5,440,146 and 5,510,626 and “A Method of Electronic Readout of Electrophotographic and Electroradiographic Images”; Journal of Applied Photographic Engineering, Volume 4, Number 4, Fall 1978, pp. 178 to 182. By the use of the electrostatic recording medium, the irradiation dose to the patient can be reduced and at the same time, the diagnostic performance can be improved.
We, this applicant, have proposed an electrostatic recording medium comprising a first conductive layer permeable to recording radiation, a recording photoconductive layer which exhibits conductivity upon exposure to the recording radiation, a charge transfer layer which behaves like a substantially insulating material to an electric charge in the same polarity as that in which the first conductive layer is charged and behaves like a substantially conductive material to the electric charge in the polarity opposite to that in which the first conductive layer is charged, a reading photoconductive layer which exhibits conductivity upon exposure to reading light and a second conductive layer permeable to the reading light which are superposed one on another in this order, and an image reading system for reading out radiation image information from the electrostatic recording medium. See, for instance, U.S. Pat. No. 6,268,614.
In the image reading system disclosed in U.S. Pat. No. 6,268,614, reading light emitted from the light source is caused to scan the electrostatic recording medium when reading out an electrostatic latent image. As a reading light exposure system which exposes the electrostatic recording medium to the reading light, there has been known a spot beam exposure means which scans the electrostatic recording medium in both the main and sub-scanning directions with a spot beam such as of a laser beam or a line beam exposure means which scans the electrostatic recording medium in only one scanning direction with a line beam. As a light source for emitting a line beam, there has been known those comprising a plurality of linearly arranged light emitting elements.
A method in which an LED array comprising a plurality of LEDs arranged like an array is employed as one of light sources comprising a plurality of linearly arranged light emitting elements has been known. (See, for instance, U.S. Patent Laid-Open NO. 20010025936.) The LED is high in efficiency versus input energy and can keep lower the cost as compared with a laser or the like. When a line light source comprising a plurality of LEDs is employed, light beams emitted from the respective LEDs are collected in a direction perpendicular to the direction in which the LEDs are arranged (will be referred to as “scanning direction”, hereinbelow) by an optical means such as a cylindrical lens disposed in parallel to the direction in which the LEDs are arranged (will be referred to as “LED arranging direction”, hereinbelow) and are projected onto the electrostatic recording medium as a reading line beam. The line beam is caused to scan the electrostatic recording medium and image information is read out from the electrostatic recording medium.
However, in such a reading light exposure system, when the light beams emitted from the respective LEDs are collected in the scanning direction by an optical means, fluctuation in focusing point is generated since the angles of divergence of the light beams emitted from the respective LEDs are not limited in the LED arranging direction. Accordingly, focused light beams and defocused light beams are mingled with each other on the electrostatic recording medium and the defocused light beams increase flares and enlarge the line width.
Further, in such a reading light exposure system, there has been a problem that the line beam on the electrostatic recording medium is not uniform in its intensity in the LED arranging direction. That is, in the line beam on the electrostatic recording medium, the central portion where light beams emitted from a larger number of LEDs are collected is higher in intensity and the intensity is reduced toward the ends of the line beam where light beams emitted from a smaller number of LEDs are collected. Such nonuniformity in intensity can deteriorate reliability of the image information read out.
In order to overcome this problem, we, this applicant, have proposed in U.S. Patent Laid-Open No. 20040046134 a reading light exposure system comprising a line source having a number of linearly arranged light emitting elements, an optical means which collects reading light beams emitted from the light emitting elements in a direction perpendicular to the light emitting element arranging direction and a pinhole array which limits the angles of divergence of the light beams emitted from the respective light emitting elements in the light emitting element arranging direction.
An example of the reading light exposure system using a pinhole array will be described, hereinbelow, with reference to FIGS. 7A and 7B. FIG. 7A is a side view of the reading light exposure system 200 as viewed in the Y-direction (the scanning direction). The LED arranging direction will be referred to as “the Z-direction” and the direction perpendicular to the Y-Z plane will be referred to as “the X-direction”, hereinbelow. FIG. 7B is a cross-sectional view taken along a plane parallel to the X-Y plane. The reading light exposure system 200 comprises a line source 101 formed by a plurality of linearly arranged surface emitting LED chips 101a, 101b, 101c . . . , a slit 102 having an opening extending in the longitudinal direction of the line source 101, a pinhole array 201 having circular pinholes 201a, 201b, 201c . . . (FIG. 8A) formed at the same intervals as the pitches at which the LED chips 101a, 101b, 101c . . . are arranged, and a pair of cylindrical lenses 104 and 105 which converge the reading light L in the Y-direction.
The slit 102 is a field stop which limits the light emitting images of the LED chips of the line source 101. The pinhole array 201 is of a predetermined thickness and limits the angles of divergence of light beams emitted from the LED chips 101a, 101b, 101c . . . to about 10° in all directions. The pinholes may be square in shape as shown in FIG. 8B.
The light emitting images of the LED chips 101a, 101b, 101c . . . are limited by the opening of the slit 102, are limited in the angles of divergence by the respective pinholes 201a, 201b, 201c . . . of the pinhole array 201, are collected in the Y-direction by the cylindrical lenses 104 and 105, and then projected onto the electrostatic recording medium 10.
Since the light distribution profile of the LED chip is very wide as shown in FIG. 9, a light beam emitted from one LED chip illuminates a very wide range unless the angle of divergence is limited in the Z-direction by the pinhole array 201. However, in the reading light exposure system 200, since the angle of divergence is narrowed in the Z-direction, the illuminating range of a light beam emitted from one LED chip is reduced to one several times to one several tens of times.
Accordingly, the number of LED chips corresponding to one illuminating spot at the central portion of the electrostatic recording medium 10 becomes smaller than the conventional. This means that a light intensity equivalent to the central portion of the electrostatic recording medium 10 can be obtained in the end portions of the electrostatic recording medium and the light intensity is reduced only in the extreme end portions of the electrostatic recording medium, whereby uniformity in the light intensity in the LED arranging direction is improved, and reliability of the image information read out can be increased.
Generally, when a reading light beam L which exits from one point and diverges in the Z-direction is converged by the cylindrical lenses 104 and 105, the reading light beam is converged on different points according to its exit angle. For example, when the electrostatic recording medium 10 is positioned on focus, the light beam is focused on the electrostatic recording medium 10 when the exit angle is 0, whereas focused on another position when the exit angle is not 0. Accordingly, focused light beams and defocused light beams are mingled with each other on the electrostatic recording medium 10, which generates flares in the linearly condensed reading light beam L and increases the line width (as measured in the Y-direction). As the angle of divergence of the reading light beam L in the Z-direction increases, flares in the linearly condensed reading light beam L becomes larger and the line width is increased. That is, in the reading light exposure system 200, flares in the reading light beam L linearly condensed on the electrostatic recording medium 10 is reduced and the line width is decreased by limiting divergence of the reading light beam L in the Z-direction, whereby reliability of the image information read out can be increased.
However, the reading light exposure system shown in FIGS. 7A and 7B gives rise to a problem that the efficiency of utilization of the reading light beam emitted from the line source is low since a pinhole array in which a number of circular or square pinholes are arranged is used. When the pinholes are increased in size, though the efficiency of utilization of the reading light beam emitted from the line source can be improved, the angle of divergence of the reading light beam in the longitudinal direction of the line source increases, which results in deterioration in uniformity of the light intensity in the longitudinal direction of the line source and increase in fluctuation of the focusing points to increase the flares, thereby deteriorating the reliability of the image information read out.
Further, the pinhole array is generally not smaller than 1 mm in thickness. When the pinhole array is several hundreds of μm in thickness, the pinhole array can be inexpensively formed by a known chemical etching technology or a known ion etching technology. However, it is difficult to form a pinhole array not smaller than 1 mm in thickness and pinhole arrays not smaller than 1 mm in thickness are generally formed by wire-cutting using electric discharge. Accordingly, the pinhole array is expensive, which naturally makes the reading light exposure system expensive.